Suppose we assume that the incidence of MI is .005 per year among participants who actually take placebo and that aspirin prevents 20% of MI (i.e. relative risk = p1/p2 = 0.8). We also assume that the duration of the study is 5 years and that the dropout rate in the aspirin group = 10% and the drop-in rate of the placebo group = 5 %.
How many participants need to be enrolled in each group to have 90% chance of detecting a significant difference using a two-sided test with a a= .05 if compliance is perfect?